The present invention relates to a fuel cell, particularly, to a liquid fuel cell adapted for miniaturization and a tank for housing a liquid fuel used for the fuel cell.
There are various types of liquid fuel cells such as liquid fuel cells of a vapor feed type, those utilizing a capillary action, and etc.
The fuel cell of the conventional vapor feed type makes it possible to use directly a liquid fuel of a high concentration and, thus, is advantageous in terms of miniaturization of the fuel section. However, since the system is complex, it is necessary to use auxiliary machines, making it difficult to miniaturize the fuel cell as it is. On the other hand, the liquid fuel cell of the conventional direct liquid feed type is adapted for miniaturization in terms of the construction of the fuel cell. However, since fuel is supplied directly in the liquid form to a fuel electrode, it is unavoidable to use fuel of a low concentration. It follows that the inner volume of a fuel tank must be enlarged, making it difficult to achieve miniaturization of the entire system.
Further, in order to take out power of the fuel cell stably, fuel is required to be supplied stably. In the conventional fuel cell, a liquid fuel is forcedly supplied to the cell body by using, for example, a liquid feed pump. However, it was difficult to supply stably a just required amount of the fuel because the conventional tank was not equipped with a mechanism for adjusting the pressure within the tank. If the pressure of the fuel tank is rendered negative, compared with the fuel cell body, it is impossible to take out further the liquid fuel from the tank, leading to a lowered power. On the other hand, where the pressure on the side of the liquid storing portion is rendered markedly high, a liquid fuel is supplied in an amount larger than that required to the fuel cell body, with the result that the fuel cell body is filled with an excessively large amount of the liquid fuel so as to markedly deteriorate the components of the fuel cell body. Alternatively, if the pressure inside the liquid storage section is markedly increased, the liquid fuel tank tends to be ruptured, which is highly dangerous. In any of the small fuel cells disclosed to the public, the liquid fuel tank is not equipped with a pressure adjusting mechanism for adjusting both the negative pressure and the excessively high pressure, which leads to the problem that it is impossible to take out the output with a high stability.
Further, a special volatile solution, e.g., a mixture of methanol and water, is used as the fuel for the fuel cell, the fuel inlet connected to the fuel cell body is required to be hermetically sealed from the outside. In the case of using a mixed fuel solution comprising liquid materials of different volatility, the composition would tend to change over time; the most volatile material evaporating first. Since the cell output is dependent on the fuel concentration, the concentration of the fuel should be maintained constant. However, if the fuel tank is hermetically sealed, especially at the inlet the volatile components of the fuel are not evaporated ensuring stable output of the fuel cell.
Particularly, where a liquid fuel is supplied to a small fuel cell, new problems are brought about. In the case of a large fuel cell, the cell need not be moved nor used in various directions. On the other hand, in the case of a small fuel cell, it is required that the cell can be used in various directions like the dry cell. In other words, it is required that a liquid fuel be supplied to the fuel cell body with a high stability in any direction of mounting the fuel cell body. This is a problem that is generated only when the cell is miniaturized. Of course, it is very important to solve this problem. Also, this problem provides a very serious obstacle that must be overcome for using a fuel cell practically as a small power source.
It is required that the fuel be taken out from the tank stably so as to obtain a stable output, and that the fuel cell has the high performance of the initial rising characteristics. Since the rising characteristics depends on the initial flow rate of the fuel from the fuel tank into the fuel-cell body, it is necessary to supply the fuel promptly to the fuel cell body. In other words, it is required that the fuel tank has a mechanism for promptly supplying the fuel in the initial period.
For getting a stable output, it is necessary that the fuel tank for the fuel cell has a following characteristics which the conventional fuel tank doesn't have. First a mechanism that prevents the liquid storage section from bearing a negative pressure or an excessively high pressure, compared with the fuel cell body. Second, a mechanism that permits the fuel to be supplied with a high stability in any direction. And last, a mechanism that permits the fuel to be supplied promptly in the initial rising period.